The present invention relates to a method and apparatus for producing nitrogen in which air is separated in a distillation column into nitrogen-rich vapor and oxygen-rich liquid. More particularly, the present invention relates to such a method and apparatus in which reflux for the distillation column is produced by condensing a stream of the nitrogen rich vapor through indirect heat exchange with a liquid stream extracted from the distillation column and thereby vaporized. Even more particularly, the present invention relates to such a method and apparatus in which the vaporized liquid stream is compressed, cooled, and introduced back into the distillation column to increase recovery. Even more particularly, the present invention relates to such a method and apparatus in which the liquid stream has a higher nitrogen content than the oxygen-rich liquid.
Nitrogen is produced from single column air separation plants known in the art as nitrogen generators. In one particularly efficient nitrogen generator, filtered, compressed and purified air is separated within a single column to produce a nitrogen-rich vapor as tower overhead and oxygen-rich liquid as column bottoms. A head condenser is provided to condense a stream of the nitrogen-rich vapor to produce column reflux. The coolant used for such condensation duty is supplied by a valve expanded stream of the oxygen-rich liquid. This coolant vaporizes and a portion of the stream is re-compressed either at the temperature of the warm or cold end of a main hem exchanger used in cooling the air to a temperature suitable for its distillation. The re-compressed, vaporized coolant is then introduced into the column in order to increase the overall recovery of the nitrogen. At the same time, part of the vaporized coolant can be partly warmed and then expanded with the performance of work. The expanded stream acts as a refrigerant to supply plant refrigeration. Part of the work of expansion is applied to the compression in order to conserve energy.
As will be discussed, the type of process, outlined above, can be operated to further increase the recovery of nitrogen product.